Historic Leaded Lights

Stephen Clare

This is an exciting time for glass
conservators in England, with several
large projects involving internationally
important glass in progress. These include
the major works by York Glaziers Trust
to York Minster’s resplendent Great East
Window by John Thornton (c1405), the 16th-century Belgian glass from Herkenrode Abbey
imported by Brooke Boothby to the Lady
Chapel at Lichfield under the care of Barley
Studio of York, and the ‘Tree of Jesse Window’
(c1340) from Wells Cathedral, presently being
conserved by Holywell Glass of Wells. Complex
interdisciplinary collaboration between glass
conservators, conservation scientists and art
historians has led to significant progress in
this field and it is not surprising that stained
glass conservation has held centre stage.

However, the majority of our heritage
in glass does not comprise such significant
painted, stained or enamelled glass, but plain
or ‘quarry’ glazing with simple, diamond-shaped
quarries of glass set in a lattice of
lead. The quality and beauty of these humble
leaded lights can be arresting, whether in a
country cottage or a medieval church. Growing
appreciation has led to greatly improved
custodianship, not least because so much has
already been damaged or replaced through
ignorance of historic methods and materials.

It is vitally important that we now pay
proper respect to the substantial amount
of plain glazing which has survived and
do not neglect the basic methods of site
fixing, maintenance and repair which
are essential to its proper care. (Many of
these principles are of course common to
both leaded lights and stained glass.)

Likewise, historic plain glazing demands
the same level of documentation as stained
glass. While there is often no need to prepare
conservation diagrams for each panel,
key areas should be carefully documented
and original glass and lead described
diagrammatically, which is straightforward
with modern computer software.

The importance of high quality, first
level training cannot be overstated. Just as
conservators cannot function with any degree
of authority without first developing high-level
craft skills, it is equally essential that
conservation awareness and sensibilities,
as well as a knowledge of historic materials
and methods, are introduced to trainees at
a very early stage irrespective of whether
their work will involve plain leaded lights or stained glass. It is equally important that
qualified highly skilled leaded light makers
and fixers are given the respect they deserve.

In the conservation and restoration of plain
glazing, the most important factor in securing
the survival of historic glazing is knowledge of
materials and appropriate techniques. Although
this can be complex, there are a few key
elements to consider which are set out below.

TRADITIONAL AND MODERN TYPES OF WINDOW GLASS

Knowledge of glass types and manufacture
is essential so that important survivals are
recognised and conserved. By understanding
the qualities of original glass – its thickness,
brilliance, texture and so on – the best
possible decisions can be made concerning
replacement when conservation is not possible.

Mass-produced sheet glass

Generally the most common types of sheet glass
throughout the 19th and 20th centuries were made by the drawn cylinder method or the Fourcault process (below left and centre).

The drawn cylinder process, developed
in America in about 1830, was basically a
mechanised and hugely expanded version of
the ancient cylinder process, which is described
later. A metal hoop was placed in the molten
glass and drawn upwards while compressed
air was blown into the glass tube. The cylinder
of glass was enormous, often drawn up to
40 feet in length before being transferred
to a cradle. Here it was cut into manageable
sections with a hot wire or diamond. These
smaller sections were then opened out into
sheets by reheating and cutting them from
one open end of the cylinder to the other, in
the same way as traditional cylinder glass.

Left: the drawn cylinder method, centre: the Fourcault Process and right: the disc of glass or ‘crown’ that gives crown glass
its name

The Fourcault process, developed in the
first years of the 20th century in Belgium,
was a continuous flat drawn sheet process. In
essence, molten glass was forced through a
slotted section of a fire-brick float sitting on
the surface of a large vat of molten glass. The
float was forced below the surface of the molten
glass to begin the process. Hydrostatic pressure
forced a flow of molten glass through the slot; it
was then gripped and drawn upwards by rollers
and cut into sheets as it solidified higher up.

It is very difficult to identify these mass
produced sheet glasses: experts will confirm
that there is a regular but subtle wave across the
sheets but this is extremely difficult to distinguish
when it has been cut into small quarries.
However, surface movement will be present
and can be picked up by careful observation.

Cylinder glass is the best replacement
for mass-produced sheet glass, but should
be selected to avoid irregularities or ‘reams’
as the cylinder glasses now available
have a largely regular surface with only
slight imperfections. Another option is
to use horticultural sheet, a type of glass
imported from Eastern Europe, which can
be successfully introduced (this is probably
still produced by the Fourcault process.)

It is important that care is taken to match
the thickness and brilliance of the original
glass, as well as carefully observing any slight
tint in the glass, which should be matched.

Machine-rolled glass

These textured glasses, often referred to as
‘cathedral’ glasses, are produced by continuous
process and passed through water-cooled
rollers to impart texture. They have been in
constant production since 1888 with frequent
changes to pattern and colour ranges. As many
variations are now obsolete, these glasses can
present difficulties to the conservator. However,
many modern textured glasses are produced
on a large scale in America and China, with
European manufacturers such as Lamberts
also producing a range of cathedral glasses.

The trade names for these ranges vary and
include ‘bulls-eye’, ‘spectrum’, ‘kokomo’ and
‘antique cathedral’. It is important to research
availability and keep abreast of changing ranges.

Because certain glass types are no longer
available, some of the conservation methods
discussed later, such as edge bonding,
copper foil repair, and the insertion of fine
repair leads, are now having to be used on
modern machine rolled glasses just to retain
original material. In effect, the ‘discard and
replace’ attitude formerly applied to machine
rolled glasses is properly under revision.

Broad glass and early cylinder glass

The traditional methods used to make both
‘white’ and coloured sheet window glass
have been in constant use since at least the
11th century. (Early glass is too strongly
tinted to be described as clear. Called ‘white’
glass, its colour is actually greenish.)

A gather of molten glass on one end of a
long blowpipe is inflated by mouth and swung
to draw the growing bulb out centrifugally
into a cylindrical shape with parallel sides.
The end furthest from the blowpipe is then
opened, usually with the help of an assistant
who pierces the end of the cylinder and
works the opening into shape as the blower
rotates the pipe. The other end is then
separated from the blowpipe by the blower.

In the earlier broad glass method, the
cylinder was then cut along a long edge and
flattened into a sheet on a stone bed while
still hot and plastic by pushing it into place
with iron and wooden tools. The marks of this manipulation are often visible.

Cylinder glass is merely a refinement of the
broad glass process. Larger cylinders are blown
and allowed to cool. They are then cut with a
diamond along their length and reheated to
form sheets, thoroughly flattened by ‘ironing’
with a heavy block of damp hardwood onto
a bed of stone, metal or glass according to
the glasshouse. It is then ‘annealed’, gradually
cooled under controlled conditions in a ‘lehr’,
a tunnel-like oven though which the glass is
slowly moved. The process removes stresses,
making the glass less likely to shatter when cut.

Broad glass production methods were
generally less sophisticated than cylinder
glass and the pieces are relatively small. The
glass varies greatly in thickness across the
sheet and for white glasses has a pronounced
greenish tint. There are also frequent bubbles
and imperfections. ‘Stones’ are often trapped
in the batch – these can be larger sand
particles from the glass constituents, or pieces
of the pot in which the glass was smelted.
Nevertheless, this glass is extremely attractive
and rich when formed into leaded lights.

The surface of broad glass is often corroded.
The fact that much of this glass is high in
potash, often with impurities present in the
batch constituents, allied to furnace technology
(which often did not achieve a homogeneous
mass in the pot) contribute to this.

Broad glass should always be conserved
and repaired as the first option. Where
replacement is necessary, cylinder glass should
be used. If the glazier’s stocks cannot match
the glass, several of the European glass houses
will match glasses with great accuracy.

A close match can normally be
found by referring to old stocks, but be
aware of the differences between the
early and modern cylinder glasses.

Old cylinder glass too should be conserved
by repair wherever possible. If replacement is
unavoidable, use modern cylinder glass. It is
still produced in the UK by English Antique
Glass Ltd, in France at the great glasshouses
of Verriers St Just, in Germany by Glasshütte
Lamberts and in Poland by Tatra Glass.

An example of 18th century crown glass plain
glazing dismantled on site following collapse.
All glass was retained and was original. Copper foil
repairs were carried out.

Crown glass

The other main historic method of sheet
glass production results in crown glass. This
has great brilliance because the glass is not
formed on a bed of stone, glass or metal so
both faces retain their smooth fire-finish.

In this process the gather of glass was blown into a large bubble, then manipulated
into the form of a wide-bottomed decanter.
A solid iron rod or ‘punty’ was then attached
to it at a point opposite the blowing iron by
dipping the punty into molten glass and fusing
it to the gather. The blowing iron was then
detached and the end of the bubble opened
out to form an aperture. This opening was then
presented to the fierce heat of the furnace until
almost molten, then spun rapidly, unfurling
into the familiar disc or crown of brilliant
glass (above left). These varied greatly in size:
medieval coloured bullions were small, only
a foot or so across, but at the height of crown
glass production in the 18th century huge
crowns were blown, often six feet across.
These became very thin at the outer edge,
often less than 1mm thick and possessing great
clarity and brilliance in these thinner areas.

As crown glass is no longer made, cylinder
glass is the only suitable replacement, but
it must be carefully selected for thickness
and tint. Specialist glaziers buy up stocks of
very thin cylinder glass when available and
will commission specific batches for projects
to ensure the best match for crown glass.
An example is the important stable block at
Stoneleigh Abbey, Warwickshire, for which
Verriers St Just blew very thin cylinders of
a particular shade of blue to special order
for Holy Well Glass, ensuring an excellent
match with surviving quarry glazing.

For sash window glazing, where the
distinct curvature of the crown is much
more visible than in the smaller pieces in
leaded light glazing, it will be necessary to
curve the glass under heat on a kiln former
to achieve the correct appearance. The
London Crown Glass Company can provide
carefully selected heat-formed curved cylinder
glass where larger amounts are required.

Production of slab glass (Diagrams
reproduced by kind permission of Peter Cormack FSA)

Slab glass

The final sort of glass commonly encountered
is Norman slab glass, which was blown into
rectangular moulds as a bottle before being cut
into small panes when cold. The thickness of
the glass, ranging from very thick at the centre
of the pane, to very thin at the edges, transmits
light in a particularly rich fashion. The use of
spectacularly coloured slabs in work of the Arts
and Crafts period is well known, but this glass
was also used to great effect in plain glazing.

For repairs, carefully selected cylinder
glass, sometimes ‘plated’ (fixing two or
more layers of glass together) to achieve the
necessary depth, can approximate slab glass.
However, there is no substitute for approaching
a conservator who holds old stocks. It is
also possible to commission new slab glass.
English Antique Glass will produce slab glass
to match the original if sufficient is ordered.

REPAIR METHODS

Resin edge bonds

While the method selected should be carefully
considered to suit the situation, in broad terms
edge bonding with epoxy resin is not generally
suitable for the repair of plain glazing. This is
because resin edge bonding is far more fragile
than other available methods of repair. It was
developed for museum use and while often
employed in the conservation of important
stained and painted glass, the bonds are
usually protected by protective glazing in some
form. Without such protection window glass
is exposed to the elements and tends to be
worked hard. Casement panels may be opened
frequently, but even fixed lights are subject to
wear and tear and local damage is common.

However, in certain situations, such as the
repair of crown glass in sash windows where
applied repair methods may be unsightly, or
where a particularly beautiful section of glass
has multiple cracks but remains intact, in
situ resin bonds may be a sensible choice.

In situ resin bonds can also be employed
in such situations as a useful temporary
repair, conserving shattered fragments in situ pending future conservation.

The crack is cleaned and degreased
by irrigating with acetone on a cotton bud
and the resin is introduced with a scalpel
tip. The resin pulls along the crack under
capillary action, which can be monitored
with an eyeglass to ensure a full flow.

To ensure that the resin cures properly it
should be measured accurately by weight on
a digital balance. The preferable epoxy resin
for this purpose is Araldite 20:20, which is
a more predictable material over a slightly
wider temperature range for in situ repair
than some other resins such as HXTAL
NYL-1. In situ resin works should not be
carried out in cold conditions or towards the
end of the working day as wide temperature
fluctuations are more likely overnight.

In studio-based conservation, resin residues
are cleaned thoroughly and all repairs marked
clearly on conservation diagrams. For in situ
bonds, however, there is an argument for leaving
the tiny bead of resin at the point of application
to serve as an indicator for future artisans
that the resin bond has been carried out.

Repair leads

Honest and reversible repair methods are
preferable for plain glazing. The insertion of
repair leads or strap leads is good practice,
conserving original material. However, there
is no reason why this sort of repair should be
clumsy and detract from the appearance of
original lead lines. There should be a hierarchy
of leads, separating original lead lines from
later repairs. Lead sections are available in
a huge variety of sizes, or can be custom
milled in-house. A sensitive, considered
approach to the insertion of repair leads is easily achieved, although often neglected.

The date 1751 and the manufacturer’s initials (WR)
were clearly visible in this original lead from the
18th century panel shown above. As much of the
original lead as possible was conserved and lead was
specially milled for the remainder.

It has been common over the centuries
for glass to be cut or ‘grozed’ (the edges
nibbled away with a notched tool or pliers)
to accommodate the ‘heart’ of the lead (the
crossbar of the H section). However, this
rather defeats the object of conserving the
original glass. It is possible to order lead from
suppliers with a very slender heart to avoid
removing glass. Alternatively, it is simple to
make a draw-tool with a sharp cutting edge
to remove shavings of lead from the heart.
Only a very small thickness is required; repair
leads usually serve the purpose of retaining
original material and are not structural, so
slim, tailor-made leads are appropriate.

There is a distinction between repair
leads inserted when a panel is dismantled
in the workshop (middle left) and ‘strap’
repairs, where a crack is covered by a
strip of surface lead applied in situ.

Strap leads should be soldered in place,
which is easily achieved with a small electric
or butane gas soldering iron. A tiny sliver
of lead is removed with a scalpel to reveal
bright metal and fluxed. The tip of a small iron
will readily solder the repair lead in place.

Historically strap leads were often applied
on both sides and ‘glued’ into place with
blackened linseed oil putty, however, unless
leads are required on both faces to stabilise
loose fragments, a lead on the exterior face
only with a little blackened putty forced under
the lead, but not into the crack, and trimmed
off neatly, can suffice. This means that the
weather is kept out, but when the panel is next
re-leaded, the trauma to the glass through
mechanical removal of hard putty is minimised.

Copper foil repairs

The copper foil or Tiffany method was
developed in America by Tiffany and
La Farge in about 1880. It has been employed
for over 30 years for the discreet repair
of broken fragments of stained glass.

A copper foil repair

In this method a very thin sheet of copper
with an adhesive backing is applied to each side
of the crack and neatly trimmed back with a
scalpel. The pieces are then brought together,
fluxed and soldered with a small soldering
iron. The same process is carried out on the
reverse, resulting in a tiny H-section repair
akin to a miniature lead. The repair can be
darkened by applying chemical patination to
make it blend in with the leaded panel, but the
solder oxidises and darkens with time anyway.

There are some justified concerns about
its use for painted and stained glass as great
care needs to be taken when trimming back the
copper foil to avoid scoring the glass surface
with the scalpel tip. The reaction of heat, fluxes
and chemical patination in close proximity to
delicate glass surfaces and painted surfaces is a
further cause for concern. Nevertheless these
issues are being addressed by conservators
and the method remains a valid option as an
honest and readily reversible repair method.

For plain glazing, the copper foil method is
an excellent choice, the scoring of the surface
can be readily avoided by careful application
and the repair is both discreet and reversible.
Because the foil is so thin there is no need to
remove any glass to facilitate the insertion.

It should be remembered that copper foil
repairs can often be carried out in situ when
fragments of a broken piece can be teased out, allowing copper foil to be applied to the edge
of cracks. Even in exposed positions when
external access may not be possible, the fact
that solder is readily drawn through to the
other side of the foil can make this viable.

This method can also be seen as an
excellent temporary repair for the retention of
original material pending future conservation.

The brilliance of slab glass

Re-leading

This is a crucial part of the process. If the wrong
decisions are taken about the lead profile, the
whole balance of a window in its architectural
setting can be destroyed. For example, Arts
and Crafts period slab glass is normally set
into substantial round-section leads. To re-lead
in thin, flat-section lead would radically
alter the external texture of the building.

In the same way, 18th-century plain glazing
formed of very thin crown glass should be
re-leaded in flat-section lead with a heart
depth matching the original. This can be as
little as 1.5mm and must be custom-milled
to ensure an authentic appearance. But it is
not merely a question of appearance: panels
from this period were normally of very light
construction and held to a very substantial
iron frame (or ‘ferramenta’) with lead ties.
The panels overlapped one another.

This dynamic can easily be upset if the
panels are made too heavy, leading to premature
failing of ties and water ingress, not to mention
an overly heavy external appearance. In plain
glazing such niceties are important – they are
the difference between success and failure.

Obviously, historic lead such as
medieval cast sections and later sections
with historically significant date markings
should be conserved wherever possible, or
at the very least thoroughly documented.

Historic Churches, 2011

Author

STEPHEN CLARE founded Holy Well Glass in
1995 having spent 15 years working with leading
stained glass design and conservation studios. He
is national adviser for stained glass to the National
Trust, a member of the Court of Assistants of the
Worshipful Company of Glaziers and trustee of The
Glaziers’ Trust, the charitable arm of the Company.